Galaxies• Galaxies are self-gravitating systems containing billions

of stars and having diameters of10–25 kpc (1 kpc = 1000 pc = 3260 lt.-yr.).

• The observed universe has billions of galaxies.

• We live in a Galaxy known as the Milky Way.

• Galaxies aren’t randomly distributed but tend to cluster.Cluster sizes range from a few dozen members, as in ourLocal Group, to 10,000 like the Virgo Cluster.

• Galaxies began forming 13 Gyr agowhen the universe was 1 Gyr old.

• Quasars may be young galaxies

• Galaxies are believed to consumeneighboring galaxies. The MilkyWay has consumed severalnearby dwarf galaxies; theMagellanic Clouds will beconsumed in the near future.

• Galaxies are classified accordingto appearance. It’s unknown ifgalaxies evolved from one classinto another.

• Our galaxy is a barred spiral.

S.H

arris

Lattimer, AST 248, Lecture 5 – p.1/15

Structure of the Milky Way• The Milky Way is a barred spiral galaxy with 200-400 billion stars and 6 · 1011 M⊙ of

stars and gas and 6 − 30 · 1011 M⊙ of dark matter.

• Major components include the nucleus (bulge), haloand disk (spiral arms.)

• The galactic center contains a supermassive black holeof 3 · 106 M⊙ called Sagittarius A. Most galaxiesare believed to have black holes at their centers.

• The galaxy’s nucleus is bar-shaped and about 27,000lt.-yrs. long, and is composed of ancient red stars.

• The galaxy’s disk contains molecular clouds, youngstars, and galactic clusters, largely concentrated in aring around the bar and in 4 major spiral arms. Mostcurrent star formation occurs here. The disk diameteris about 100,000 lt.-yrs.

• Spiral arms are logarithmic spirals (r = aebθ), just likethe spirals in a nautilus shell or a hurricane.

• The Sun is located at the inner rim of the Orion Arm,25,000 lt.-yrs. from the galactic center.

• The galactic halo is a spheroid of old stars and globularclusters with a diameter estimated at 200,000 lt.-yrs.The galaxy’s dark matter is uniformly distributedwithin the halo.

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Lattimer, AST 248, Lecture 5 – p.2/15

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• Many stars in the disk are in open clusters.

• Many stars in the halo are in globular clusters.

• Molecular clouds are the birthplaces of stars andcontain gas (atoms and molecules) and dust grains.

• The material between stars (99% gas, 1% dust) comprisesthe interstellar medium (15% of visible mass of Galaxy).

• 75% by mass of gas is H, 23% is He. Many moleculesare observed, most complex ones are organic.

• Dust is composed ofheavy elements andices (H2O, CO2).

• Gas and dustobscure starlight.

• Some gas reflectslight, some radiateslight and radio waves.

• Radio emissions areused to map galacticmotions and structure.

Lattimer, AST 248, Lecture 5 – p.4/15

Observed Interstellar MoleculesSimple Hydrides, Oxides, Suldes, Halides, and Related Mole ulesH2 CO NH3 CS NaCl N2OHCl SiO SiH4 SiS AlClH2O SO2 CC H2S KClHNO OCS CH4 PN AlFNitriles, A etylene Derivatives, and Related Mole ulesHCN HC C|CN H3C|CC|CN H3C|CH2|CN H2C=CH2H3CCN H(CC)2|CN H3C|C CH H2C=CH|CN HCCHCCCO H(CC)3|CN H3C(CC)2| H HNC H3CNCCCCS H(CC)4|CN H3C(CC)2| CN? HN=C=O HCCNCHCCCHO H(CC)5|CN? HN=C=S CCO C5OCCC CCCCC C4Si NaCNAldehydes, Al ohols, Ethers, Ketones, Amides, and Related Mole ulesH2C=O H3COH HO|CH=O H2CNH H2C=C=OH2C=S H3C|CH2|OH H3C|O|CH=O H3CNH2 H2C=C=CH3C|CH=O H3CSH H3C|O|CH3 H2NCN H2C=C=C=C(CH3)2CO? NH2|CH=O NH2|CH2|C|O|OHCy li Mole ulesC3H2 SiC2 C3H PAHsIonsCH+ HCO+ HCNH+ H3O+ HCS+ HN+2CO+ HOCO+ HC3NH+ HOC+ H2D+ SO+Radi alsOH C3H CN HCO C2S HCOCH2CH C4H C3N NO NSNH C5H H2CCN SO SiCCH2 C6H H2CN MgCN SiNC2H C8H HCCN2 MgNC CP

Spin- ip radiation from hydrogen atoms. The state where the spins ofthe proton and ele tron are aligned has a slightly higher energy than thestate where the spins are opposed. The energy hange between the twostates is E = h = h =, leading to a frequen y of = 1420 MHz or awavelength of = = = 21:1 m.

Implications of abundantorganic molecules in ISM:Formed in harsh conditions.Organic molecules seem to bepreferred over other varieties.How complex can they get?Complexity should be greater incomets, planetary atmospheres,planetary surfaces.PolyAromatic Hydrocarbons (PAHs)and amino acids found in meteorites.

Could living molecules or viruses form?Could life travel in comets or meteoritesfrom planet-to-planet or star-to-star(panspermia)?

Lattimer, AST 248, Lecture 5 – p.5/15

Big Bang and CosmologyHubble discovered in the 1920’s that the universeis expanding. This suggests that the universebegan with a Big Bang. Main evidence today:

• Redshifts (z = v/c) of galaxies increase withdistance: Hubble’s Law: v = H D

Hubble’s constant is H ≃ 50 km/sec/Mpc. Noterelation to D = vt which gives the age of the

universe as t = D/v = H−1≈ 20 Gyr.

Universe has not expanded at a constant rate,so the true age is about 14 Gyr.

• Microwave background radiation with a uniformT = 2.7 K (Predicted by Alpher, Herman andGamow in 1948 and discovered by Penzias andWilson in 1965, who received 1978 Nobel Prize).

This originated at the decoupling era when the uni-verse was 300,000 yrs old and hot (T ≈ 3000 K).

• Cosmic nucleosynthesis of light elements (He, Li,Be, B) when the universe was seconds old andhotter (T ≈ 109 K). Produced composition of 75%H and 25% He by mass, observed in oldest stars.

WMAPLattimer, AST 248, Lecture 5 – p.6/15

S. Harris

Lattimer, AST 248, Lecture 5 – p.7/15

Future of the UniverseDepends upon density d:

Ω = d/dc

dc = 3H2/8πG ≈ 3 H atoms/m3 (critical density)

• Ω > 1 (d > dc): closed universe, bound,eventually recontracts.

• Ω < 1 (d < dc): open universe, unbound, expandsforever.

• Ω = 1 (d = dc): critical case, expansion stalls, butno recontraction. This case is mathematicallyelegant, and solves the so-called flatness problem.If Ω is not exactly 1, then Ω should either be muchmuch less than 1 or much much greater than 1 atpresent. Also solves horizon problem, which is whyis the background radiation so uniform in all directions?

Observed abundances of light nuclei (1H1, 2He4,

2He3, 1H2, 4Li7) agree with theory if the densityof ordinary baryonic matter (neutrons, protons andnuclei) is about 4% of the critical density(Ωbaryon ≈ 0.04). Another way of saying this is

that the baryon-to-photon ratio is about 10−6.

Lattimer, AST 248, Lecture 5 – p.8/15

Density of the UniverseBut measurements of our Galaxy’s mass andmotions of galaxies in clusters imply a largeamount of “missing mass” or “dark matter”(Ωdark matter ≈ 0.26) that is not in the formof baryons and what form it has not is as yetunknown. Moreover, observations of

• cosmic background fluctuations

• very distant Type I supernovae

imply that

• the universe’s expansion is now accelerating,not decelerating as gravity alone would cause

• the total Ω is 1 (i.e., the universe is critical).

This implies the existence of yet another form ofmass-energy, the so-called “dark energy”, withΩdark energy ≈ 0.7. ThereforeΩdark energy + Ωdark matter + Ωbaryon = 1.Dark energy is not matter and no one knows whatit could be; one suggestion is that it is the exoticmatter that was proposed for faster-than-light travel.

Nick Strobel www.astronomynotes.com

NASA

Lattimer, AST 248, Lecture 5 – p.9/15

Missing Mass

S. Harris

NASA/CXC/CIA/STSci/Magellan/Univ. of Ariz./ESO Lattimer, AST 248, Lecture 5 – p.10/15

Lattimer, AST 248, Lecture 5 – p.11/15

InflationConceptual problems with the Big Bang:

• Horizon: why is background radiation uniform when itoriginates from regions that are now causallydisconnected?

• Flatness: why is Ω so close to 1, when it could havebeen any number?

• Smoothness: where did perturbations in nearlyuniform universe originate? Without these perturba-tions, stars and galaxies would never have formed.

• Monopole: Magnets have two poles: why not one?

• Antimatter: Why is universe predominately matter?

Apparent Solution: Early period of rapid growth (inflation)• About 10−36 seconds after birth, parts of the universe underwent massive

expansions similar to weak parts of inflating balloon.

• These regions expanded so much they now appear flat (Ω = 1).

• All observable matter was causally connected before inflation, so it maintainsuniform temperature today after inflation.

• Perturbations needed for structures inflated from quantum irregularities inpre-inflation universe.

T. Knott, Wikipedia

Lattimer, AST 248, Lecture 5 – p.12/15

History of the Universe

www.damtp.cam.ac.uk/user/gr/public

Lattimer, AST 248, Lecture 5 – p.13/15

Cosmic Coincidence or Fine Tuning

• P.A.M. Dirac pointed out in 1937 that ratio of electrical to gravitational forces in Hatom is about 1039. This is the same of the size of the observable universe to thesize of the proton! Why? Must gravity be weakening in order to keep thesenumbers the same, since the universe is expanding?

• R. Dicke suggested an anthropic argument: The lifetimes of stars is controlled bythe ratio of the electrical and gravitational forces, but the size of the universe by thetime since the Big Bang. Therefore, the size of the universe at the time WE canobserve it is controlled by the same ratio since stellar lifetime is needed forbiological evolution.

• Other coincidences or examples of fine-tuning abound:• Excited state in C12 at exactly right level for triple-α process (He fusion) to

synthesize C and get around the fact that no stable elements of mass 5 or 8exist.

• Strength of the nuclear weak force controls the neutrino flows that causesupernova explosions, which are necessary to create heavy elements.Otherwise, black holes would always form, trapping the heavy elementswithin.

Lattimer, AST 248, Lecture 5 – p.14/15

Cosmic Anthropic Principle• We must take into account the selection effects

imposed by our status as living observers.

• Fundamental constants of nature G, me, mp, mn, e, ~, c

have values that permit existence of intelligence. Is thischance or necessity? (Weak Anthropic Principle)• Perhaps constants are related through a Grand

Unified Theory• Perhaps life is not so dependent upon fine-tuning

of these constants

• The Universe must have properties that allow life todevelop at some stage (Strong Anthropic Principle):• Are observers necessary for universe to exist?• Are there multiple universes, so that there is a finite

chance that intelligence can develop in one of them?

• Suggests that intelligence is rare and maybe unique.

• Carter deduces that if biological evolution andstellar evolution times are not related, life is rare.

• Will intelligence last forever once it comes intoexistence? (Final Anthropic Principle)

• Participatory Anthropic Principle: we participatein creation of universe (Quantum Entanglement).

Lattimer, AST 248, Lecture 5 – p.15/15